1. Field of the Invention
The present invention relates generally to method and apparatus for handling waste and, more particularly, to method and apparatus for treating a water solution of a waste material containing a salt having smelt-water explosion characteristics.
2. Description of the Prior Art
It has been known that explosions can occur in the situation where a mass of a molten or fused sodium or potassium chloride is dipped or submerged at a high temperature into water. The explosion of this class is called "smelt-water explosion". The salt of the kind specified is called "salt having smelt-water explosion characteristics". A salt of the class specified above is contained in waste produced in the pulp industry, for example.
Japanese Post-Examination Patent Publication No. 51-38185 (published Oct. 20, 1976), No. 52-13673 (published Apr. 15, 1977) and No. 52-36354 (published Sep. 14, 1977) are all concerned with method and apparatus for treating a water solution of a waste material containing a salt having smelt-water explosion characteristics. The apparatus disclosed in these Japanese publications comprises an incinerator and a gas-liquid separator. The incinerator defines therein an open-bottomed combustion chamber and is provided with at least one spray nozzle through which the waste solution is sprayed into the combustion chamber. The incinerator is also provided with a substantially downwardly directed fuel burner for producing a combustion of a fuel in the combustion chamber to heat the spray of the solution to a temperature at which the water in the sprayed solution is vaporized and the salt contained in the sprayed solution is fused. Any organic component of the waste material is burned away in the combustion chamber. The combustion gases and the water vapor thus produced flow through the combustion chamber toward the open bottom thereof. At least a part of the fused salt forms a deposit on the inner surface of the combustion chamber and flows toward the open bottom.
The gas-liquid separator comprises a substantially closed vessel provided with an inlet substantially vertically aligned with the open bottom of the combustion chamber in the incinerator, a gas outlet and a liquid outlet formed in the vessel and disposed at positions remote from the inlet in generally horizontal direction. The open bottom of the combustion chamber and the inlet of the gas-liquid separator vessel are connected by a substantially vertical cylindrical duct or annular wall. The fused salt flows downwardly on the inner surface of the annular wall. Cooling water is introduced through the annular wall into contact with a part of the surface of the fused salt flowing on the inner surface of the wall so that the salt is cooled and at least partially solidified and cracked into separate masses which fall into the gas-liquid separator vessel. A part of the cooling water thus introduced is vaporized and the rest of the water falls down into the gas-liquid separator vessel. The combustion gases and the water vapor flow in the vessel in generally horizontal direction toward the gas outlet.
The masses of salt are further cooled in the separator vessel by water and dissolved in the cooling water, so that the salt can be recovered in the form of an aqueous solution thereof.
In the Japanese publications referred to above, references are made to the mechanism of the "smelt-water explosion". It is stated that the mechanism of the smelt-water explosion has not been exactly known but is believed to be due to the fact that, when a mass of a fused salt having smelt-water explosion characteristics is dipped at a high temperature into and surrounded by water, the water permeates into the mass of the fused salt. Simultaneously, the fused salt is cooled to form a relatively hard, smooth and continuous gas-tight skin or shell in the surface area of the mass of the salt with a result that the water which has permeated into the salt is trapped within the hard "shell". The inner part of the mass of the salt, however, is still at a temperature high enough to vaporize the trapped water. The vaporization of the water causes an abrupt pressure build-up within the hard "shell" resulting in an explosion. In addition to sodium chloride and potassium chloride referred to above, sodium hydroxide and sodium sulfide are also known as being inorganic salts which have the smelt-water explosion characteristics. Sodium carbonate and sodium sulfate are also inorganic salts, but they are known as being free from the smelt-water explosion. The reason is believed to be that the skins or shells of the masses of fused sodium carbonate and sulfate formed when they are dipped into and cooled by water are softer than the shells of the masses of salts having the smelt-water explosion characteristics and thus allow the vaporized water to easily break out and flow from the softer shells without causing an explosion.
In the apparatus disclosed in Publication No. 52-13673, the partially solidified and cracked masses of fused salt fall directly onto the bottom of the gas-liquid separator vessel together with a part of the cooling water introduced through the annular wall. Thus, a bath of the water is formed in the vessel to a certain depth. The masses of salt which fall onto the vessel bottom are dipped in and contacted by the water. In order to avoid explosion, therefore, the apparatus must be arranged such that the water supply is so controlled as to assure that, by the time the masses of salt fall into the water bath, they are sufficiently cooled to a temperature at which no explosion takes place, i.e., a temperature at which no rapid vaporization of the water trapped within the masses of salt occurs.
The Japanese Publication No. 52-13673 also teaches that, even in the case where a mass of fused sodium or potassium chloride is cooled by direct contact with water, the possibility of dangerous smelt-water explosion can be minimized if the mass is dipped in water not entirely or wholly but only partially. The reason is believed to be that the water vapor produced from the water trapped in the mass of fused salt can flow out of the mass through that portion of the mass which is not in contact with the water and thus is still soft enough to permit the passage of the water vapor.
In the apparatus disclosed in the Japanese Publication No. 51-38185 and No. 52-36354, a perforated horizontal partition is provided in the gas-liquid separator vessel to divide the interior thereof into upper and lower spaces for the gases and liquid, respectively, so that masses of salt which fall on the partition are prevented from being dipped in the bath of the cooling water formed on the bottom of the separator vessel. A conical "dispersion" member is disposed on the horizontal partition in vertical alignment with the annular wall so that the cooling water which falls into the gas-liquid separator is rapidly dispersed therein radially outwardly.